The present invention is related to cells in a rack for receiving fuel assemblies, and more particularly to cells having protruding corners which facilitate construction of the rack.
Nuclear fuel is supplied to both pressurized water reactors and boiling water reactors in the form of elongated fuel assemblies. A fuel assembly comprises a bundle of fuel rods formed by filling hollow cylinders with pellets of fissionable fuel enriched with U-235, together with control rods and other structures. Fuel assemblies are commercially available in various dimensions; a typical fuel assembly for a boiling water reactor has a length of 175 inches (4.45 meters), a square cross-section of 301/4 square inches (195 square centimeters), and a mass of 600 pounds (270 kilograms).
After a service life of several years in a reactor, the U-235 enrichment of a fuel assembly is depleted. Furthermore, a variety of fission products, having various half-lives, are present in the fuel rods. These fission products generate intense radioactivity and heat when the fuel assemblies are removed from the reactor, and accordingly the fuel assemblies removed for short-term storage to a spent fuel rack that is submerged in a pool containing boron salts dissolved in water. Although this rack is deemed a "spent fuel" rack, it may also be used to temporarily store fuel assemblies which have a remaining service life and which will be returned to the reactor. Because of this, the rack should offer lateral support to the fuel assemblies so that their weight does not distort them and make return to the reactor impossible.
FIG. 1A illustrates a conventional spent fuel rack 20 for holding the typical BWR fuel assemblies mentioned above. Although only a corner of rack 20 is illustrated, the rack provides a checkerboard array of 10.times.10 storage slots for receiving one hundred fuel assemblies. Rack 20 includes a stainless steel base plate 22 having one hundred holes 24 in it to permit circulation of water. Fifty cells (of which only cells 26, 28, and 30 are illustrated) are affixed to plate 22 and are joined to each other, in a manner to be described, so that fifty of the storage slots are provided within the fifty cells. The remaining fifty storage slots are provided between the walls of four adjacent cells and will be deemed "free" storage slots to distinguish them from a storage slot within a cell. Since four adjacent cells are not available at the periphery of plate 22, the "free" storage slots in this region are completed by panels that are connected to base 22 and a pair of peripheral cells. For example, in FIG. 1A panel 32 is welded to cells 26 and 30 and to plate 22 to complete the "free" storage slot bounded by cells 26-30.
The construction of a conventional cell will be described with reference to FIG. 1B, wherein cell 28, for example, is an elongated hollow structure having four stainless steel walls 34. A sheet of neutron poison 36, such as boron carbide, is disposed on each wall and retained in place by a stainless steel wrapper 38, which is welded to the wall 34. Neutron poison sheets 36 are present to prevent criticality by isolating the fuel assemblies housed in rack 20. At the four corners of cell 28, the walls 34 may be joined by butt-welding, as at 40, or by bent portions (the same as bent portion 42 of cell 26).
The dimensions of the hundred storage slots provided by rack 20 must match those of the particular fuel assemblies which are to be stored, so that the fuel assemblies fit snugly within the storage slots and receive lateral support from the sides thereof. The lateral support keeps the fuel assemblies from bowing unduly, which would prevent their return to the reactor. A snug fit is particularly important in view of the possibility of an earthquake, since the lateral support provided by the cells must keep the stored fuel assemblies from rattling around unduly during a seismic disturbance. For the typical fuel assemblies mentioned above the cells are 172 inches (437 cm) high and have inside dimensions of 6.025 inches (15.30 cm) from one wall 34 to the opposite wall 34. The radius of curvature of portions 42 must be 1/4 inch (0.64 cm) or less, and is typically 0.062 inch (0.16 cm) at the inside surface. A short radius of curvature is necessary because fuel assemblies have sharp edges which might be damaged during a seismic event if bent portions 42 of larger radius were used. Moreover the use of a short radius of curvature lessens the risk that a stored fuel assembly might rotate slightly during installation or during a seismic event. Such rotation might cause the fuel assembly to bind with the cell and be difficult or impossible to remove.
If the outer corners of such cells were welded directly together it will be apparent that poison sheets 36 and wrappers 38 would protrude into the space available for the "free" storage slots, making them too small. In order to avoid this problem FIG. 1B illustrates a rod 44 secured between the corners of cells 26 and 28 by welds 46. For the typical fuel assemblies mentioned above this provides a center-to-center spacing of 6.25 inches (15.88 cm) between a storage slot provided within a cell and the nearest "free" storage slots. Like all things mechanical, however, in practice the dimensions of the cells may deviate slightly, and manufacturing tolerances are established.
Rack 20 is constructed from the center outward. Construction starts by inserting a square bottom fixture into a central hole 24, positioning the bottom of a cell using this fixture, inserting another square fixture at the top of the cell to ensure proper orientation with respect to plate 22, welding the bottom of the cell to plate 22, and then removing the fixtures. Square bottom fixtures are then inserted into the holes 24 at the four corners of the just-installed central cell, and four cells are positioned between the bottom fixtures and top fixtures. While the fixtures maintain the cells in their proper positions, technicians weld the bottoms of the cells to plate 22, select rods 44 of the appropriate diameter to bridge the gaps between the corners of the cells, and weld the selected rods 44 to the cells. The fixtures are then removed and work begins on the next ring of cells.
The width of the gap between the corners of adjacent cells before the rods 44 are installed is determined by the dimensional variance of the particular cells. In extreme situations there may be no gap at all and the cells are welded directly together. Moreover, in general a rod need not be welded along the entire height of the cells in order to ensure that rack 20 is sufficiently stable to withstand possible seismic disturbances which may be encountered during use. That is to say, the minimum length of the weld varies depending upon the geology at particular reactor sites. The technicians must measure the lengths of the welds they apply in view of the seismic requirements.
Construction of a spent fuel rack 20 using conventional cells in this manner is cumbersome for several reasons. In addition to the base plate 22 and cells, a supply of rods 44 must be kept on hand during fabrication of the rack. Furthermore the need to select rods and weld them into the gaps between cells significantly increases the work of the technicians who make a rack. The fabrication process is further encumbered because the technicians must measure the lengths of the welds.